The information displayed by a display device, such as a liquid crystal display device, is difficult to read in low light conditions. This is why lighting such display devices by means of a light source was considered very early on. The most basic known solutions consist simply in arranging a point light source, such as a filament bulb or light emitting diode, at the back of the liquid crystal display device. Such solutions are, however, unsatisfactory since they do not ensure homogenous illumination of the display surface of the liquid crystal display device. Indeed, a more intense halo of light is perceptible to the user through the liquid crystal display device at the place where the point light source is located.
To overcome this problem, it has been proposed to illuminate liquid crystal display devices by means of a light guide. A light guide is a component of limited thickness made of a transparent material, typically a plastic material, whose dimensions substantially match those of the liquid crystal display device to be illuminated and into which is injected the light produced by at least one light source optically coupled to the light guide. Optical microstructures called light extractors, whose function is to uniformly extract, across the entire surface of the light guide, the light injected into the light guide by the light source, are structured in at least one of the upper or lower surfaces of the light guide.
An example of a transparent light guide of the type briefly described above is represented in FIG. 1, annexed to the present Patent Application. Designated as a whole by the general reference number 1, the light guide is, for example, of generally parallelepiped shape and comprises an upper surface 2 and a lower surface 4 connected to each other by a lateral edge 6. It also comprises one or more holes 8 for the mounting thereof onto a support piece (not shown) and a pair of studs 10 for securing a printed circuit sheet 12 serving as support for a point light source 14, such as a light emitting diode.
Light guide 1, which is made of transparent plastic material, comprises optical prisms 16 commonly called light extractors, which, in the example represented in FIG. 1, are structured in upper surface 2 of light guide 1. The purpose of these light extractors 16 is to upwardly extract the light injected into light guide 1 by point light source 14 optically coupled to light guide 1. Such a light guide 1 is thus intended to light from below a display device, such as a liquid crystal display cell (not shown).
In order for the illumination of the display device to be as bright and homogenous as possible, any leakage of light must therefore be prevented as far as possible. It has been observed that, in the particular case where a light guide is used to light from below a liquid crystal display device housed inside a watch case, the light that leaks through the sides of the light guide may be reflected onto some of the watch components and diffused upwards towards the watch dial. Undesired light spots thus appear which impair the aesthetic appearance of the watch.
To overcome this problem, upper surface 2 of light guide 1 is divided into a peripheral surface 18 and a useful surface 20 which substantially matches the surface of the display device to be illuminated and is surrounded by peripheral surface 18. As represented in FIG. 1, peripheral surface 18 is coated with a layer 22 of a light absorbing material, such as a black paint, in order to prevent the light produced by light source 14 from leaking at that location. Likewise, studs 10 for securing printed circuit sheet 12, which serves as a support for point light source 14, are integral with light guide 1 and are therefore transparent. They should also be coated with light absorbing paint. Finally, again for the purpose of limiting light leakage as far as possible, lateral edge 6 may be chamfered. Indeed, as it is difficult to deposit absorbent paint on lateral edge 6 of light guide 1, the edge is chamfered. It is clear that chamfering lateral edge 6 reduces the surface through which light can escape laterally.
The operations intended to reduce light leakage in a light guide of the type described above are, however, difficult to implement on an industrial scale. Layer 22 of absorbent material is typically deposited by pad printing, which is long and complex especially as it must be ensured that light guide 1 is precisely positioned when layer 22 of absorbent material is deposited. Indeed, care must be taken not to deposit absorbent material on useful surface 20 of light guide 1, through which escapes the light intended to illuminate the display device. Moreover, chamfering lateral edge 6 does not completely prevent light leakage.